The invention relates in general to drive mechanisms and in particular to a new and useful servo drive for valves, slide valves or the like having a planetary gear train construction.
Servo drives with such a planetary gear train, referred to as returning, which is designed as a two-step drive are in use with two internally toothed outer gear wheels, the first of which is also externally toothed and is in engagement with a worm mounted in a gearcase, and the second of which is connected with a carrier plate secured on the output shaft of the gear train, further with two interconnected externally toothed inner wheels which are driven by an eccentric.
A planetary gear train constructed according to the same principle is known in the trade under the designation "The Keystone epi". This planetary gear train has an externally toothed inner wheel mounted on an eccentric by a rolling bearing, and an external toothing, consisting of two toothed rims of different diameters, and likewise has only one internally and externally toothed outer wheel, the external toothing of which meshes with a worm secured on the shaft of a manual drive, while its internal toothing meshes with one of the two toothed rims of the inner wheel. The second rim of the inner wheel is in engagement with the internal toothing of a bowl-shaped carrier plate, which is secured on the output shaft of the gear train and in which the internal toothing is on the inner face of a cylinder ring shaped wall perpendicular to the plate or disc.
In the above described planetary gear trains of the prior art, the inner wheels are, due to their construction, more or less smaller in diameter than the internal toothing of the outer wheels.
In single step gear trains the transmission ratio u is defined by the ratio of the difference between the number z.sub.1 of teeth of the internal toothing of the outer wheel acting directly on the output shaft and the number z.sub.2 of the teeth of the external toothing of the inner wheel mounted on the eccentric to the number z.sub.2 of teeth of the external toothing of the inner wheel, according to the equation ##EQU1##
The above described commercially known two-step gear train, in which the outer wheel is prevented from rotating, has, according to Dubbel, Taschenbuch fur den Maschinenbau (Manual of Machine Construction), Berlin, published by Julius Springer, 1935, page 717, a transmission ratio ##EQU2## where r.sub.1 =radius of the internal toothing of the bowl-shaped carrier plate,
r.sub.2 =radius of the larger toothed rim of the inner wheel,
r.sub.3 =radius of the smaller toothed rim of the inner wheel,
r.sub.4 =radius of the internal toothing of the outer wheel.
The radius ratio r.sub.1 /r.sub.2 and r.sub.3 /r.sub.4 in pairs of meshing wheels is the same as the ratio z.sub.1 /z.sub.2 or z.sub.3 /z.sub.4 of the number of teeth of the respective gears, so that the above relationship for J=1/u corresponds to: ##EQU3##
For two-step gear trains it is preferably taken to be large, e.g. in the range of 100&lt;1/u&lt;1000.
Also in a planetary gear train according to the above considered prior art, where the cylinder ring shaped wall with internal toothing of the carrier plate may be conceived as replacing a second outer wheel, as shown in FIG. 2, the transmission ratio is large because of the small difference therebetween the diameters of the two rims of the externally toothed inner wheel.